Bevan Smith, of Vogeltown, asks :-

I understand that the Earth's magnetic field is not static, but changes direction with time. The position of the North and South poles varies from year to year. Volcanic extrusions on the floor of the oceans have shown a recurring reversal of direction of magnetism over the aeons. Over what length of time do the poles reverse, and when is the next reversal likely to happen? What effect if any will this have on the Earth's magnetosphere, and any influence on surface conditions?

Tony Hurst, a geophysicist with GNS Science, responded.

The most rapid reversals seem to only take a few hundred years. Only 41,000 years ago, there was a very short reversal, known as the Laschamp Event, in which it took about 250 years for the field to reverse, but it only stayed reversed for about 500 years before returning to its normal polarity equally rapidly.

There is an upper limit as to how fast reversals can occur, even if the electrical currents in the earth's outer core reversed almost instantaneously (which is extremely unlikely), it would take something like 50 years for the effects to reach the surface and the magnetosphere above us.

During the reversal, the magnetic field is significantly weaker than normal, and very variable. The main effect of this is that it is not so efficient at deflecting cosmic rays. We can see this effect by greater quantities of the isotope Be10 being contained in the sediments produced during the reversal.

Some authors have suggested that more plant and animal species than normal become extinct during reversals, but as we see more reversals any correlation seems weaker. One way in which the increased cosmic rays could affect species on the earth's surface is by production of an Ozone Hole, something mankind started to accidentally produce.

“The human race has survived many excursions and a few reversals already: so we are likely to come through the next one unscathed.”

The above is a fair statement of the unlikelihood of our biological processes being affected by a reversal. However, many aspects of our technological society could be influenced by more variable magnetic fields during a reversal damaging power grids, or by disturbances to satellite orbits, and maybe other effects we have not thought of. This suggests unpleasantness and societal difficulties, but not something beyond our ability to cope with.

As far as when the next reversal will occur, since accurate measurements of the strength of the earth's magnetic field began about 500 years ago, the strength has dropped about 5% each century, as shown in this diagram from the British Geological Survey.

At this rate, a reversal might occur in 2000 years. However, we do not know whether such a decrease in field strength typically precedes a reversal.